In the prior art, an ultra-smooth sheet can be fabricated by laminating a plastics film on one face of a paper, the plastics film defining an ultra-smooth face on the paper. The base paper is made of a fibrous material and its faces present relatively large roughness, of the order of about 20 micrometers (μm), i.e. each of its faces has projections and indentations with the height between them being of the order of 20 μm. Laminating a plastics film on one of the faces of such a paper serves to impart very small roughness to that face, roughness of the order of 1 μm when using a polyethylene terephthalate (PET) film.
Since paper is a material that is relatively expensive and that is produced on a large scale, it is important for it to be recyclable. Nevertheless, a paper-based sheet that is ultra-smooth because it includes a plastics film is not recyclable or is difficult to recycle, which is therefore neither ecological nor inexpensive. When recycling paper-based sheets, the sheets are chopped up and mixed with water in a pulper in order to form a paper pulp. When such sheets include plastics films, those films are torn up in the pulper and their plastics material pollutes the pulp.
With the present state of the art, it is therefore not possible to fabricate an ultra-smooth sheet that is recyclable, and preferably entirely recyclable.
Furthermore, such an ultra-smooth sheet is not printable and a printable resin needs to be deposited on the plastics film of the sheet in order to make it printable. That technique is used in particular for fabricating paper-based sheets for printing photographic images (known as “resin-coated photographic papers”), where such sheets have a polyethylene (PE) film with Bekk smoothness of about 6000 seconds (s).
A smooth sheet may also be fabricated by depositing a coating layer on one face of a paper, with that composition, once dry, defining a smooth face on the paper. That technique makes it possible to fabricate a smooth sheet without a plastics film. The composition is deposited on the paper by a curtain coating technique, with a scraper or trailing blade, with an air knife, by photogravure, or by rollers (size press, film press, etc.). The face of the base paper onto which the coating composition is deposited has alternating depressions and projections, the depressions being filled by the coating composition and the projections being made even during coating, thereby enabling the roughness of the paper to be reduced. Nevertheless, that technique does not enable a sheet to be obtained that is as smooth as a sheet covered in a plastics film, even if the sheet is subsequently smoothed, e.g. by calendering.
The method presently used for fabricating a smooth and glossy sheet consists in depositing a coating composition on a base paper by means of a mechanical roller having a cylindrical surface that is very smooth and that is covered in a layer of chromium. The Bekk smoothness of a sheet obtained by that method is of the order of 50 s and is therefore nevertheless smaller than that of a sheet having a plastics film (which is about 6000 s when a PE film is applied).
Furthermore, it is difficult to obtain a smooth sheet by coating a composition onto a paper that is relatively rough. When the above-mentioned depressions in the face of the paper are too large or too numerous, the coating composition does not fill the depressions completely, or else too great a quantity of composition is required to do that.
This applies for example to a paper having relatively large bulk, e.g. greater than 1.10 cubic centimeters per gram (cm3/g), which therefore has faces that are relatively rough and poor printability. Coating a composition on a face of such paper, even using large quantities of composition, does not enable a smooth sheet to be fabricated, and also considerably reduces its bulk. Furthermore, even though calendering the sheet makes it possible to increase its smoothness, that is achieved to the detriment of its bulk.
In the prior art, it is therefore not possible to fabricate, under satisfactory conditions, a smooth sheet starting from a rough paper and/or having relatively large bulk.